RELATIONSHIP OF THE TRILOBITES TO OTHER ARTHROPODA.
It can not be said that the new discoveries of appendagiferous
trilobites have added greatly to previous knowledge of the systematic
position of the group. Probably none will now deny that trilobites are
Crustacea, and more primitive and generalized than any other group in
that class. The chief interest at present lies in their relation to
the most nearly allied groups, and to the crustacean ancestor.
Trilobites have been most often compared with Branchiopoda, Isopoda,
and Merostomata, the present concensus of opinion inclining toward the
notostracan branchiopods (Apodidæ in particular) as the most closely
allied forms. It seems hardly worth while to burden these pages
with a history of opinion on this subject, since it was not until the
appendages were fully made out, from 1881 to 1895, that zoologists and
palæontologists were in a position to give an intelligent judgment.
The present status is due chiefly to Bernard (1894), Beecher (1897,
1900, et seq.), and Walcott (1912, et seq.).
The chief primitive characteristics of trilobites are: direct
development from a protaspis common to the subclass; variability in
the number of segments, position of the mouth, and type of eyes; and
serially similar biramous appendages.
The recent study has modified the last statement slightly, since it
appears that in some trilobites there was a modification of the
appendages about the mouth, suggesting the initiation of a set of
tagmata.
In comparing the trilobites with other Crustacea, the condition of the
appendages must be especially borne in mind, for while these organs
are those most intimately in contact with the environment, and most
subject to modification and change, yet they have proved of greatest
service in classification.
Appendages have been found on trilobites from only the Middle Cambrian
and Middle and Upper Ordovician, but as the Ordovician was the time of
maximum development of the group, it is probable that trilobites of
later ages would show degradational rather than progressive changes.
All the genera which are known show appendages of the same plan, and
although new discoveries will doubtless reveal many modifications of
that plan, general inferences may be drawn now with some assurance.
The chief characteristics of the appendages are: first, simple
antennules, a primitive feature in all Crustacea, as shown by
ontogeny; second, paired biramous appendages, similar to each other
all along the body, the youngest and simplest in front of the anal
segment, the oldest and most modified on the cephalon. The endobases
are retained on all the coxopodites, except possibly, in some species,
the anterior ones, and these gnathobases are modified in some genera
as mouth-parts, while in others they are similar throughout the
series. With these few fundamentals in mind, other Crustacea may be
examined for likenesses. The differences are obvious.
Crustacea.
BRANCHIOPODA.
The early idea that the trilobites were closely related to the
Branchiopoda was rejuvenated by the work of Bernard on the Apodidæ
(1892) and has since received the support of most writers on the
subject. Fundamentally, a great deal of the argument seems to be that
_Apus_ lies the nearest of any modern representative of the class to the
theoretical crustacean ancestor, and as the trilobites are the oldest
Crustacea, they must be closely related. Most writers state that the
trilobites could not be derived from the Branchiopoda (see, however,
Walcott 1912 A), nor the latter from any known trilobite, but both
subclasses are believed to be close to the parent stem.
Viewed from the dorsal side, there is very little similarity between
any of the branchiopods and the trilobites, and it is only in the
Notostraca, with their sessile eyes and depressed form, that any
comparison can be made. The chief way in which modern Branchiopoda and
Trilobita agree is that both have a variable number of segments in
the body, that number becoming very large in _Apus_ on the one hand and
_Mesonacis_ and _Pædeumias_ on the other. In neither are the appendages,
except those about the mouth, grouped in tagmata. Other likenesses
are: the Branchiopoda are the only Crustacea, other than Trilobita, in
which gnathobases are found on limbs far removed from the mouth; the
trunk limbs are essentially leaf-like in both, though the limb of the
branchiopod is not so primitive as that of the trilobite; caudal cerci
occur in both groups.
If the appendages be compared in a little more detail, the differences
prove more striking than the likenesses.
In the Branchiopoda, the antennules are either not segmented or only
obscurely so. In trilobites they are richly segmented.
In Branchiopoda, the antennæ are variable. In the Notostraca they are
vestigial, while in the males of the Anostraca they are powerful and
often complexly developed claspers. Either condition might develop
from the generalized biramous antennas of Trilobita, but the present
evidence indicates a tendency toward obsolescence. Claus' observations
indicate that the antennæ of the Anostraca are developments of the
exopodites, rather than of the endopodites.
The mandibles and maxillæ of the Branchiopoda are greatly reduced, and
grouped closely about the mouth. Only the coxopodites of the Trilobita
are modified as oral appendages.
The trunk limbs of _Apus_ are supposed to be the most primitive among
the Branchiopoda, and comparison will be made with them. Each
appendage consists of a flattened axial portion, from the inner margin
of which spring six endites, and from the outer, two large flat exites
(see fig. 34). This limb is not articulated with the ventral membrane,
but attached to it, and, if Lankester's interpretation of the origin
of schizopodal limbs be correct, then the limb of _Apus_ bears very
little relation to that of the Trilobita. In _Apus_ there is no
distinct coxopodite and the endobases which so greatly resemble the
similar organs in the Trilobita are not really homologous with them,
but are developments of the first endite. Beecher's comparison of the
posterior thoracic and pygidial limbs of _Triarthrus_ with those of
_Apus_ can not be sustained. Neither _Triarthrus_ nor any other
trilobite shows any trace of phyllopodan limbs. Beecher figured (1894
B, pl. 7, figs. 3, 4) a series of endopodites from the pygidium of a
young _Triarthrus_ beside a series of limbs from a larval _Apus_.
Superficially, they are strikingly alike, but while the endopodites of
_Triarthrus_ are segmented, the limbs of _Apus_ are not, and the parts
which appear to be similar are really not homologous. The similarity
of the thoracic limbs in the two groups is therefore a case of
parallelism and does not denote relationship.
Geologically, the Branchiopoda are as old as the Trilobita, and while
they did not have the development in the past that the trilobite
had, they were apparently differentiated fully as early. Anostraca,
Notostraca and Conchostraca, three of the four orders, are represented
in the Cambrian by forms which are, except in their appendages, as
highly organized as the existing species. Brief notes on the principal
Middle Cambrian Branchiopoda follow:
=Burgessia bella= Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 177,
pl. 27, figs. 1-3; pl. 30, figs. 3, 4.
This is the most strikingly like the modern Branchiopoda of any
species described by Walcott from the Middle Cambrian, and invites
comparison with _Apus_. The carapace is long, loosely attached to the
body, and extends over the greater part of the thorax. The eyes are
small, sessile, and close to the anterior margin.
The appendages of the head consist of two pairs of antennæ, and three
pairs of slender, jointed legs. Both pairs of antennæ are slender and
many-jointed, the antennules somewhat smaller than the antennæ. The
exact structure of the limbs about the mouth has not yet been made
out, but they are slender, tapering, endopodite-like legs, with at
least three or four segments in each, and probably more.
There are eight pairs of thoracic appendages, each limb having the
form of the endopodite of a trilobite and consisting of seven segments
and a terminal spine. The proximal three segments of each appendage
are larger than the outer ones, and have a flattened triangular
expansion on the inner side. Walcott also states that "One specimen
shows on seven pairs of legs, small, elongate, oval bodies attached
near the first joint to the outer side of the leg. These bodies left
but slight impression on the rock and are rarely seen. They appear to
represent the gills." They are not figured, but taken in connection
with the endopodite-like appearance of the segmented limbs, one would
expect them to be vestigial exopodites.
A small hypostoma is present on the ventral side, and several of the
specimens show wonderfully well the form of the alimentary canal and
the hepatic cæca. The main branches of the latter enter the mesenteron
just behind the fifth pair of cephalic appendages.
Behind the thorax the abdomen is long, limbless, and tapers to a
point. It is said to consist of at least thirty segments.
Compared with _Apus_, _Burgessia_ appears both more primitive and more
specialized. The carapace and limbless abdomen are _Apus_-like, but
there are very few appendagiferous segments, and the appendages are
not at all phyllopodan, but directly comparable with those of
trilobites, except, of course, for the uniramous character of the
cephalic limbs. A closer comparison may be made with _Marrella_.
=Waptia fieldensis= Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 181,
pl. 27, figs. 4, 5.
The carapace is short, covering the head and the anterior part of the
thorax. The latter consists of eight short segments with appendages,
while the six abdominal segments, which are similar to those of the
thorax, are without limbs except for the last, which bears a pair
of broad swimmerets. The eyes are marginal and pedunculate. The
antennules are imperfectly known, but apparently short, while the
antennas are long and slender, with relatively few, long, segments.
The mandibles appear to be like endopodites of trilobites and show
at least six segments. As so often happens in these specimens from
British Columbia, the preservation of the other appendages is
unsatisfactory. As illustrated (Walcott, 1912 A, pl. 27, fig. 5), both
endopodites and exopodites appear to be present, and the shaft of the
exopodite seems to be segmented as in _Triarthrus_.
Walcott considers _Waptia_ as a transitional form between the
Branchiopoda and the Malacostraca.
=Yohoia tenuis= Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 172,
pl. 29, figs. 7-13.
This species, though incompletely known, has several interesting
characteristics. The head shows, quite plainly in some specimens, the
five segments of which it is composed. The eyes are small, situated in
a niche between the first and second segments, and are described as
being pedunculate. The eight segments of the thorax all show short
triangular pleural extensions, somewhat like those of _Remopleurides_
or _Robergia_. The abdomen consists of four cylindrical segments, the
last with a pair of expanded caudal rami.
The antennules appear to be short, while the antennas are large, with
several segments, ending in three spines, and apparently adapted for
serving as claspers in the male. The third, fourth, and fifth pairs of
cephalic appendages are short, tapering, endopodite-like legs similar
to those of _Burgessia_.
The appendages of the thorax are not well preserved, and there seem to
be none on the abdomen.
This species is referred by Walcott to the Anostraca.
=Opabina regalis= Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 167,
pl. 27, fig. 6; pl. 28, fig. 1.
This most remarkably specialized anostracan is not well enough known
to allow comparison to be made with other contemporaneous Crustacea,
but it is worthy of mention.
There is no carapace, the eyes are pedunculated, thorax and abdomen
are not differentiated, and the telson is a broad, elongate, spatulate
plate. There seem to be sexual differences in the form of the anterior
cephalic and caudal appendages, but this is not fully established. The
most remarkable feature is the long, large, median cephalic appendage
which is so suggestive of the proboscis of the recent _Thamnocephalus
platyurus_ Packard. The appendages are not well enough preserved to
permit a determination as to whether they are schizopodal or
phyllopodan.
_Summary._
Walcott referred _Burgessia_ and _Waptia_ to new families under the
Notostraca, while _Yohoia_ and _Opabina_ were placed with the
Anostraca. Except for the development of the carapace, there is a
striking similarity between _Waptia_ and _Yohoia_, serving to connect
the two groups.
The Branchiopoda were very highly specialized as early as Middle
Cambrian time, the carapace of the Notostraca being fully developed
and the abdomen limbless. Some (_Burgessia_) had numerous segments,
but most had relatively few. The most striking point about them,
however, is that so far as is known none of them had phyllopodan
limbs. While the preservation is in most cases unsatisfactory, such
limbs as are preserved are trilobite-like, and in the case of
_Burgessia_ there can be no possible doubt of the structure. Another
interesting feature is the retention by _Yohoia_ of vestiges of
pleural lobes. The Middle Cambrian Branchiopoda are more closely
allied to the Trilobita than are the modern ones, but still the
subclass is not so closely related to that group as has been thought.
Modern _Apus_ is certainly much less like a trilobite than has been
supposed, and very far from being primitive. The Branchiopoda of the
Middle Cambrian could have been derived from the trilobites by the
loss of the pleural lobes, the development of the posterior margin of
the cephalon to form a carapace, and the loss of the appendages from
the abdominal segments. Modern branchiopods can be derived from those
of the Middle Cambrian by the modification of the appendages through
the reduction of the endopodite and exopodite and the growth of the
endites and exites from the proximal segments.
Carpenter (1903, p. 334), from his study of recent crustaceans, has
already come to the conclusion that the Branchiopoda are not the most
primitive subclass, and this opinion is strengthened by evidence
derived from the Trilobita and from the Branchiopoda of the Middle
Cambrian.
COPEPODA.
The non-parasitic Eucopepoda are in many ways much nearer to the
trilobites than any other Crustacea. These little animals lack the
carapace, and the body is short, with typically ten free segments and
a telson bearing caudal furcæ. The head is composed of five segments
(if the first thoracic segment is really the fused first and second),
is often flattened, and lacks compound eyes. Pleural lobes are well
developed, but instead of being flattened as in the trilobite, they
are turned down at the sides or even incurved. A labrum is present.
The antennules, antennæ, and mandibles are quite like those of
trilobites. The antennules are very long and made up of numerous
segments. The antennæ are biramous, the junction between the
coxopodite and basipodite is well marked, and the endopodite consists
of only two segments.
The mandibles are said to "retain more completely than in any other
Crustacea the form of biramous swimming limbs which they possess in
the nauplius." The coxopodites form jaws, while both the reduced
endopodite and exopodite are furnished with long setæ. The maxillulæ
are also biramous, but very different in form from those of the
trilobite, and the maxillæ are phyllopodan.
The first thoracic limb is uniramous and similar to the maxillæ, but
the five following pairs are biramous swimming legs with coxopodite,
basipodite, exopodite, and endopodite. Both the exopodite and
endopodite are shorter than in the trilobites, but bear setæ and
spines.
The last pair of thoracic limbs are usually modified in the male into
copulatory organs. In some females they are enlarged to form plates
for the protection of the eggs, in others they are unmodified. In
still others they are much reduced or disappear. The abdomen is
without appendages.
The development in Copepoda is direct, by the addition posteriorly to
the larval form (nauplius) of segments, and the appendages remain
nearly unmodified in the adult.
Altogether, the primitive Copepoda seem much more closely allied to
the Trilobita than any other modern Crustacea, but unfortunately no
fossil representative of the subclass has been found. This is not so
surprising when one considers the habits and the habitat of most of
the existing species. Many are parasitic, many pelagic in both fresh
and marine waters, and many of those living on the bottom belong to
the deep sea or fresh water. Most free-living forms are minute, and
all have thin tests.
The eyes of copepods are of interest, in that they suggest the paired
ocelli of the Harpedidæ and Trinucleidæ. In the Copepoda there are, in
the simplest and typical form of these organs, three ocelli, each
supplied with its own nerve from the brain. Two of these are dorsal
and look upward, while the third is ventral. In some forms the dorsal
ocelli are doubled, so that five in all are present (cf. some species
of Harpes with three ocelli on each mound). In some, the cuticle over
the dorsal eyes is thickened so as to form a lens, as appears to be
the case in the trilobites. These peculiar eyes may be a direct
inheritance from the Hypoparia.
ARCHICOPEPODA.
Professor Schuchert has called my attention to the exceedingly curious
little crustacean which Handlirsch (1914) has described from the
Triassic of the Vosges. Handlirsch erected a new species, genus,
family, and order for this animal, which he considered most closely
allied to the copepods, hence the ordinal name. _Euthycarcinus
kessleri_, the species in question, was found in a clayey lens in the
Voltzia sandstone (Upper Bunter). Associated with the new crustacean
were specimens of _Estheria_ only, but in the Voltzia sandstone itself
land plants, fresh and brackish water animals, and occasionally,
marine animals are found. The clayey lens seems to have been of fresh
or brackish water origin.
All of the specimens (three were found) are small, about 35 mm. long
without including the caudal rami, crushed flat, and not very well
preserved. The head is short, not so wide as the succeeding segments,
and apparently has large compound eyes at the posterior lateral
angles. The thorax consists of six segments which are broader than the
head or abdomen. The abdomen, which is not quite complete in any one
specimen, is interpreted by Handlirsch as having four segments in the
female and five in the male. Least satisfactory of all are traces of
what are interpreted by the describer as a pair of long stiff
unsegmented cerci or stylets on the last segment.
The ventral side of one head shield shows faint traces of several
appendages which must have presented great difficulty in their
interpretation. A pair of antennules appear to spring from near the
front of the lower surface, and the remainder of the organs are
grouped about the mouth, which is on the median line back of the
center. Handlirsch sees in these somewhat obscure appendages four
pairs of biramous limbs, antennæ, mandibles, maxillulæ, and maxillæ,
both branches of each consisting of short similar segments,
endopodites and exopodites being alike pediform.
Each segment of the thorax has a pair of appendages, and those on
the first two are clearly biramous. The endopodites are walking legs
made up of numerous short segments (twelve or thirteen according to
Handlirsch's drawing), while the exopodite is a long breathing and
rowing limb, evidently of great flexibility and curiously like the
antennules of the same animal. The individual segments are narrow at
the proximal end, expand greatly at the sides, and have a concave
distal profile. A limb reminds one of a stipe of _Diplograptus_.
Both branches are spiniferous.
No appendages are actually present on the abdomen, but each segment
has a pair of scars showing the points of attachment. From the small
size of these, it is inferred that the limbs were poorly developed.
This species is described in so much detail because, if it is a
primitive copepod, it has a very important bearing on the ancestry of
that group and is the only related form that has been found fossil.
The non-parasitic copepods have typically ten (eleven) free segments,
including the telson, and the four abdominal segments are much more
slender than the six in front of them. In this respect the agreement
is striking, and the presence of five pairs of appendages in the head
and six free segments in the thorax is a more primitive condition than
in modern forms where the first two thoracic segments are apparently
fused (Calman, 1909, p. 73).
The large compound eyes of this animal are of course not present in
the copepods, but as vestiges of eyes have been found in the young of
_Calanus_, it is possible that the ancestral forms had eyes.
The greatest difficulty is in finding a satisfactory explanation of
the appendages. The general condition is somewhat more primitive than
in the copepods, for all the appendages are biramous, while in the
modern forms the maxillipeds are uniramous and the sixth pair of
thoracic appendages are usually modified in the male as copulatory
organs. In the copepods the modification is in the direction of
reduction, both endopodites and exopodites usually possessing fewer
segments than the corresponding branches in the trilobites. The
endopodite of _Euthycarcinus_, on the contrary, possesses, if
Handlirsch's interpretation is correct, twice as many segments as the
endopodite of a trilobite. If the Copepoda are descended from the
trilobites, as everything tends to indicate, then _Euthycarcinus_ is
certainly not a connecting link. The only truly copepodan
characteristic of this genus is the agreement in number and
disposition of free segments. The division into three regions instead
of two, the compound eyes, and the structure of the appendages are all
foreign to that group.
With the Limulava fresh in mind, one is tempted to compare
_Euthycarcinus_ with that ancient type. The short head and large
marginal eyes recall _Sidneyia_, and the grouping of the appendages
about the mouth also suggests that genus and _Emeraldella_. In the
Limulava likewise there is a contraction of the posterior segments,
although it is behind the ninth instead of the sixth. There is no
likeness in detail between the appendages of the Limulava and those of
_Euthycarcinus_, but the composite claws of _Sidneyia_ show that in
this group there was a tendency toward the formation of extra
segments.
If this fossil had been found in the Cambrian instead of the Triassic,
it would probably have been referred to the Limulava, and is not
at all impossible that it is a descendant from that group. As a
connecting link between the Trilobita and Copepoda it is, however,
quite unsatisfactory.
OSTRACODA.
The bivalved shell of the Ostracoda gives to this group of animals an
external appearance very different from that of the trilobites, but
the few appendages, though highly modified, are directly comparable.
The development, although modified by the early appearance of the
bivalved shell within which the nauplius lies, is direct. Imperfect
compound eyes are present in one family.
The antennules are short and much modified by functioning as swimming,
creeping, or digging organs. They consist of eight or less segments.
The antennas are also locomotor organs, and in most orders are
biramous. The mandibles are biramous and usually with, but sometimes
without, a gnathobase. The maxillulæ are likewise biramous but much
modified.
The homology of the third post-oral limb is in question, some
considering it a maxilla and others a maxilliped. It has various forms
in different genera. It is always much modified, but exopodite and
endopodite are generally represented at least by rudiments. The fourth
post-oral limb is a lobed plate, usually not distinctly segmented, and
the fifth a uniramous pediform leg. The sixth, if present at all, is
vestigial.
Very little comparison can be made between the Ostracoda and
Trilobita, other than in the ground-plan of the limbs, but the
presence of biramous antennæ is a primitive characteristic.
CIRRIPEDIA.
Like the ostracod, the adult cirriped bears little external
resemblance to the trilobite. The form of the nauplius is somewhat
peculiar, but it has the typical three pairs of appendages, to which
are added in the later metanauplius stages the maxillæ and six pairs
of thoracic appendages. In the adult, the antennules, which serve for
attachment of the larva, usually persist in a functionless condition,
while the antennas disappear. The mandibles, maxillulæ, and maxillæ
are simple and much modified to form mouth parts, and the six pairs of
thoracic appendages are developed into long, multisegmented, biramous
appendages bearing numerous setæ which serve for catching prey. Paired
eyes are present in later metanauplius stages, but lost early in the
development. The relationship to the trilobite evidently is not close.
MALACOSTRACA.
_1. Phyllocarida._
The oldest malacostracans whose appendages are known are species of
_Hymenocaris_. One, described as long ago as 1866 by Salter, has what
seem to be a pair of antennæ and a pair of jaw-like mouth-parts.
Another more completely known species has recently been reported by
Walcott (1912 A, p. 183, pl. 31, figs. 1-6). This latter form is
described as having five pairs of cephalic appendages: a pair of
minute antennules beside the small pedunculated eyes, a pair of large
uniramous antennæ, slender mandibles and maxillulæ, and large maxillæ
composed of short stout segments. There are eight pairs of biramous
thoracic limbs, the exopodites setiferous, the endopodites composed of
short wide segments and ending in terminal claw-like spines. These
appendages are like those of trilobites.
_Hymcnocaris_ belongs to the great group of extinct ceratocarid
Crustacea which are admitted to the lowest of the malacostracan
orders, Phyllocarida, because of their resemblance to _Nebalia_,
_Paranebalia_, _Nebaliopsis_, and _Nebaliella_, the four genera which
are at present living. The general form of the recent and fossil
representatives of the order is strikingly similar. The chief outward
difference is that in many of the fossils the telson is accompanied by
two furcal rami, while in the modern genera it is simple. It now
becomes possible to make some comparison between the appendages of
_Hymcnocaris_ of the Middle Cambrian and the Nebaliidæ of modern seas.
In both there are five pairs of cephalic and eight of thoracic
appendages, while those of the abdomen of Hymenocaris are not known.
In both, the antennules are less developed than the antennæ. In the
Nebaliidæ the antennules show evidence of having been originally
double (they are obviously so in the embryo), while they are single in
_Hymcnocaris_. In both, the antennæ are simple. The remaining cephalic
organs are too little shown by the specimen from the Middle Cambrian
to allow detailed comparison. The mandibles, maxillulæ, and maxillæ of
_Nebalia_ are, however, of types which could be derived from the
trilobite.
In three of the genera of the Nebaliidæ, the eight pairs of thoracic
limbs are all similar to one another, though those of the genera
differ. All are biramous. The limbs of _Hymcnocaris_ can apparently be
most closely correlated with those of _Nebalia antarctica_, in which
the endopodite consists of short flattened segments, and the exopodite
is a long setiferous plate. Epipodites are present in both _Nebalia_
and _Hymcnocaris_.
So far as the appendages of _Hymenocaris_ are known, they agree very
well with those of the Nebaliidæ, and since they are of the trilobite
type, it may safely be stated that the Trilobita and Malacostraca are
closely related.
_2. Syncarida._
Walcott (1918, p. 170) has compared the limbs of _Neolenus_ with those
of the syncarid genera _Anaspides_ and _Koonunga_. These are primitive
Malacostraca without a carapace, but as they have a compressed test
and _Anaspides_ has stalked eyes, their gross anatomy does not suggest
the trilobite. The thoracic appendages are very trilobite-like, since
the endopodite has six segments (in _Anaspides_) and a multisegmented
setiferous exopodite. The coxopodites, except of the first thoracic
segment, do not, however, show endobases, and those which are
present are peculiar articulated ones. The cephalic appendages are
specialized, and the antennules double as in most of the Malacostraca.
External epipodites are very numerous on the anterior limbs.
This group extends back as far as the Pennsylvanian and had then
probably already become adapted to fresh-water life. It may be
significant that the Palæozoic syncarids appear to have lacked
epipodites. While differing very considerably from the Trilobita, the
Syncarida could have been derived from them.
_3. Isopoda._
Since the earliest times there has been a constant temptation to
compare the depressed shields of the trilobites with the similar ones
of isopods. Indeed, when _Scrolls_ with its Lichadian body was first
discovered about a hundred years ago, it was thought that living
trilobites had been found at last. The trilobate body, cephalic
shield, sessile eyes, abdominal shield, and pleural extensions make a
wonderful parallel. This similarity is, however, somewhat superficial.
The appendages are very definitely segregated in groups on the various
regions of the body, and while the pleopods are biramous, the thoracic
legs are without exopodites (except in very early stages of
development of one genus). The Isopoda arose just at the time of the
disappearance of the Trilobita, and there seems a possibility of a
direct derivation of the one group from the other. It should be
pointed out that while the differences of Isopoda from Trilobita are
important, they are all of a kind which could have been produced by
the development from a trilobite-like stock. For example:
Isopoda have a definite number of segments. There is less variation in
the number of segments among the later than the earlier trilobites.
Isopoda have no facial suture. In at least three genera of trilobites
the cheeks become fused to the cranidium and the sutures obliterated.
Isopoda have one or two segments of the thorax annexed to the head.
While this is not known to occur in trilobites, it is possible that it
did.
Most Isopoda have a fairly stiff ventral test. The ventral membrane of
trilobites would probably have become stiffened by impregnation of
lime if the habit of enrollment had been given up.
In Isopoda the antennæ are practically uniramous sensory organs. The
second cephalic appendages of trilobites are capable of such
development through reduction of the exopodite.
In the Isopoda the coxopodites are usually fused with the body,
remaining as free, movably articulated segments only in a part of the
thoracic legs of one suborder, the Asellota. Endobases are entirely
absent. This is of course entirely unlike the condition in Trilobita,
but a probable modification.
In Isopoda there is a distinct grouping of the appendages, with
specialization of function. The trilobites show a beginning of
tagmata, and such development would be expected if evolution were
progressive.
In both groups, development from the embryo is direct. Rudiments of
exopodites of thoracic legs have been seen in the young of one genus.
The oldest known isopod is _Oxyuropoda ligioides_ Carpenter and
Swain (Proc. Royal Irish Acad., vol. 27, sect. B, 1908, p. 63,
fig. 1), found in the Upper Devonian of County Kilkenny, Ireland. The
appendages are not known, but the test is in some ways like that of a
trilobite. The thorax, abdomen, and pygidium are especially like those
of certain trilobites, and there is no greater differentiation between
thorax and abdomen than there is between the regions before and behind
the fifteenth segment of a _Pædeumias_ or _Mesonacis_. The anal
segment is directly comparable to the pygidium of a _Ceraurus_, the
stiff unsegmented uropods being like the great lateral spines of that
genus.
The interpretation of the head offered by Carpenter and Swain is very
difficult to understand, as their description and figure do not seem
to agree. What they consider the first thoracic segment (fused with
the head) seems to me to be the posterior part of the cephalon and it
shows at the back a narrow transverse area which is at least analogous
to the nuchal segment of the trilobite. If this interpretation can be
sustained, _Oxyuropoda_ would be a very primitive isopod in which the
first thoracic segment (second of Carpenter and Swain) is still free.
According to the interpretation of the original authors, the species
is more specialized than recent Isopoda, as they claim that two
thoracic segments are fused in the head. The second interpretation was
perhaps made on the basis of the number of segments (nineteen) in a
recent isopod.
=Marrella splendens= WALCOTT.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192,
pls. 25, 26.
Among the most wonderful of the specimens described by Doctor
Walcott is the "lace crab." While the systematic position was not
satisfactorily determined by the describer, it has been aptly compared
to a trilobite. The great nuchal and genal spines and the large
marginal sessile eyes, coupled with the almost total lack of thoracic
and abdominal test, give it a bizarre appearance which may obscure its
real relationships.
The cephalon appears to bear five pairs of appendages, antennules, and
antennæ, both tactile organs with numerous short segments, mandibles,
and first and second maxillæ. The last three pairs are elongate, very
spinose limbs, of peculiar appearance. They seem to have seven
segments, but are not well preserved. These organs are attached near
the posterior end of the labrum.
There are twenty-four pairs of biramous thoracic appendages, which
lack endobases. The endopodites are long and slender, with numerous
spines; the exopodites have narrow, thin shafts, with long, forward
pointed setæ. The anal segment consists of a single plate.
Further information about this fossil will be eagerly awaited. None of
the illustrations so far published shows biramous appendages on the
cephalon. This, coupled with the presence of tactile antennæ, makes
its reference to the Trilobita impossible, but the present
interpretation indicates that it was closely allied to them.
[Illustration: Fig. 32. _Marrella splendens_ Walcott. Restoration of
the ventral surface, based upon the photographs and descriptions
published by Walcott. Although all the limbs of the trunk appear to be
biramous, only endopodites are placed on one side and exopodites on
the other, for the sake of greater clearness in the illustration.
Drawn by Doctor Elvira Wood, under the supervision of the writer.
× about 6.]
_Restoration of Marrella._
(Text fig. 32.)
The accompanying restoration of the ventral surface of _Marrella_ is a
tentative one, based on Doctor Walcott's description and figures. The
outline is taken from his plate 26, figure 1; the appendages of the
head from plate 26, figures 1-3, 5, and plate 25, figures 2, 3; the
endopodites, shown on the left side only, from figures 3 and 6, plate
25. I have not studied actual specimens, and the original description
is very incomplete. The restoration is therefore subject to revision
as the species becomes better known.
Arachnida.
No attempt will be made to pass in review all of the subclasses of the
arachnids. Some of the Merostomata are so obviously trilobite-like
that it would seem that their relationship could easily be proved. The
task has not yet been satisfactorily accomplished, however, and new
information seems only to add to the difficulties.
So far as I know, the Araneæ have not previously been compared
directly with trilobites, although such treatment consists merely in
calling attention to their crustacean affinities, as has often been
done.
Carpenter's excellent summary (1903, p. 347) of the relationship of
the Arachnida to the trilobites may well be quoted at this point:
The discussion in a former section of this essay on the
relationship between the various orders of Arachnida led to the
conclusion that the primitive arachnids were aquatic animals,
breathing by means of appendicular gills. Naturally, therefore, we
compare the arachnids with the Crustacea rather than with the
Insecta. The immediate progenitors of the Arachnida appear to have
possessed a head with four pairs of limbs, a thorax with three
segments, and an abdomen with thirteen segments and' a telson, only
six of which can be clearly shown by comparative morphology to have
carried appendicular gills. But embryological evidence enables us
to postulate with confidence still more remote ancestors in which
the head carried well developed compound eyes and five pairs of
appendages, while it may be supposed that all the abdominal
segments, except the anal, bore limbs. In these very ancient
arthropods, all the limbs, except the feelers, had ambulatory and
branchial branches; and one important feature in the evolution of
the Arachnida must have been the division of labour between the
anterior and posterior limbs, the former becoming specialized for
locomotion, the latter for breathing. Another was the loss of
feelers and the degeneration of the compound eyes. Thus we are led
to trace the Arachnida (including the Merostomata and Xiphosura)
back to ancestors which can not be regarded as arachnids, but which
were identical with the primitive trilobites, and near the
ancestral stock of the whole crustacean class.
TRILOBITES NOT ARACHNIDA.
While no one having any real knowledge of the Trilobita has adopted
Lankester's scheme of the inclusion of the group as the primitive
grade in the Arachnida, reference to it may not be amiss. This theory
is best set forth in the Encyclopædia Britannica, Eleventh Edition,
under the article on Arachnida. It is there pointed out that the
primitive arachnid, like the primitive crustacean, should be an animal
without a fixed number of somites, and without definitely grouped
tagmata. As Lankester words it, they should be anomomeristic and
anomotagmatic. The trilobites are such animals, and he considers them
Arachnida and not Crustacea for the following reasons:
Firstly and chiefly, because they have only one pair (apart from the
eyes) of pre-oral appendages. "This fact renders their association
with the Crustacea impossible, if classification is to be the
expression of genetic affinity inferred from structural coincidence."
Secondly, the lateral eyes resemble no known eyes so closely as the
lateral eyes of _Limulus_.
Thirdly, the trilobation of the head and body, due to the expansion
and flattening of the sides or pleura, is like that of _Limulus_, but
"no crustacean exhibits this trilobite form."
Fourthly, there is a tendency to form a pygidial or telsonic shield,
"a fusion of the posterior somites of the body, which is precisely
identical in character with the metasomatic carapace of _Limulus_." No
crustacean shows metasomatic fusion of segments.
Fifthly, a large post-anal spine is developed "in some trilobites" (he
refers to a figure of _Dalmanites_).
Sixthly, there are frequently lateral spines on the pleura as in
_Limulus_. No crustacean has lateral pleural spines.
These points may be taken up in order.
1. If trilobites have one appendage-bearing segment in front of the
mouth, they are Arachnida; if two, Crustacea. This is based on the
idea that in the course of evolution of the Arthropoda, the mouth has
shifted backward from a terminal position, and that as a pair of
appendages is passed, they lose their function as mouth-parts and
eventually become simple tactile organs. Thus arise the cheliceræ of
most arachnids, and the two pairs of tactile antennæ of most
Crustacea. This theory is excellent, and the rule holds well for
modern forms, but as shown by the varying length of the hypostoma in
different trilobites, the position of the mouth had not become fixed
in that group. In some trilobites, like _Triarthrus_, the gnathobases
of the second pair of appendages still function, but in all, so far as
known, the mouth was back of the points of attachment of at least two
pairs of appendages, and in some at least, back of the points of
attachment of four pairs. As pointed out in the case of _Calymene_ and
_Ceraurus_, the trilobites show a tendency toward the degeneration of
the first and second pairs of biramous appendages, particularly of the
gnathobases. They are in just that stage of the backward movement of
the mouth when the function of the antennæ as mandibles has not yet
been lost. If the presence of functional gnathobases back of the
mouth, rather than the points of attachment in front of the mouth, is
to be the guide, then Triarthrus might be classed as an arachnid and
_Calymene_ and _Isotelus_ as crustaceans. In other words, the rule
breaks down in this primitive group.
2. Superficially, the eyes of some trilobites do look like those of
_Limulus_, but how close the similarity really was it is impossible to
say. The schizochroal eyes were certainly very different, and Watase
and Exner both found the structure of the eye of the trilobite unlike
that of _Limulus_.
3. The importance of the trilobate form of the trilobite is very much
overestimated. It and the pygidium are due solely to functional
requirements. The axial lobe contained practically all the vital
organs and the side lobes were mechanical in origin and secondarily
protective. That the crustacean is not trilobate is frequently
asserted by zoologists, yet every text-book contains a picture of a
segment of a lobster with its axial and pleural lobes. It is a
fundamental structure among the Crustacea, obscured because most of
them are compressed rather than depressed.
4. The pygidium of trilobites is compared with the metasomatic shield
of _Limulus_. No homology, if homology is intended, could be more
erroneous. The metasomatic shield of _Limulus_ is, as shown by
ontogeny and phylogeny, formed by the fusion of segments formerly
free, and includes the segments between the cephalic and anal shields,
or what would be known as the thorax of a trilobite. No trilobite
has a metasomatic shield. The pygidium of a trilobite, as shown by
ontogeny, is built up by growth in front of the anal region, and since
the segments were never free, it can not strictly be said to be
composed of fused segments. Some Crustacea do form a pygidial shield,
as in certain orders of the Isopoda.
5. The post-anal spine of Dalmanites and some other trilobites is
similar to that of _Limulus_, but this seems a point of no especial
significance. That a similar spine has not been developed in the
Crustacea is probably due to the fact that they do not have the broad
depressed shape which makes it so difficult for a _Limulus_ to right
itself when once turned on its back. Relatively few trilobites have
it, and it is probably correlated with some special adaptation.
6. There is nothing among the trilobites comparable to the movable
lateral spines of the metasoma of _Limulus_.
While, as classifications are made up, the Trilobita must be placed in
the Crustacea rather than the Arachnida, there is no reason why both
the modern Crustacea and the Arachnida should not be derived from the
trilobites.
MEROSTOMATA.
It has been a custom of long standing to compare the trilobite with
_Limulus_. Packard (1872) gave great vitality to the theory of
the close affinity of the two when he described the so-called
trilobite-stage in the development of _Limulus polyphemus_. His
influence on Walcott's ideas (1881) is obvious. Lankester has gone
still further, and associated the Trilobita with the Merostomata in
the Arachnida.
The absence of antennules at any stage in development allies _Limulus_
so closely with the Arachnida and separates it so far from the
Trilobita that in recent years there has been a tendency to give up
the attempt to prove a relationship between the merostomes and
trilobites, especially since Clarke and Ruedemann, in their extensive
study of the Eurypterida, found nothing to indicate the crustacean
nature of that group. A new point of view is, however, presented by
the curious _Sidneyia inexpectans_ and _Emeraldella brocki_ described
by Walcott from the Middle Cambrian.
=Sidneyia inexpectans= Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 21,
pl. 2, fig. 1 (not figs. 2, 3); pls. 3-5; pl. 6, fig. 3; pl. 7,
fig. 1.
The body of this animal is elongate, somewhat eurypterid-like, but
with a broad telson supplied with lateral swimmerets. The cephalon is
short, with lateral compound eyes. The trunk consists of eleven
segments, the anterior nine of which are conspicuously wider than the
two behind them, and the telson consists of a single elongate plate.
On the ventral side of the head there is a large hypostoma and five,
pairs of appendages. The first pair are multisegmented antennules. The
second pair have not been adequately described. The third are large,
complex claws, and the fourth and fifth suggest broad, stocky
endopodites. Broad gnathobases are attached to the coxopodites of the
third to fifth pairs of appendages and form very strong jaws.
The first nine segments of the thorax have one pair each of broad
filiform branchial appendages, suggestive of the exopodites of
trilobites, but no endopodites have been seen. The tenth and eleventh
segments seem to lack appendages entirely.
=Emeraldella brocki= Walcott.
Illustrated: _Sidneyia inexpectans_ Walcott _partim_, Smithson.
Misc. Coll., vol. 57, 1911, pl. 2, figs. 2, 3 (not fig. 1);--Ibid.,
1912, p. 206, text fig. 10.
_Emeraldella brocki_ Walcott, Ibid., 1912, p. 203, pl. 30, fig. 2;
text fig. 8;--Ibid., vol. 67, 1918, p. 118 (correction).
_Emeraldella_ has much the same shape as _Sidneyia_ and the same
number of segments, but instead of a broad flat telson, it has a long
_Limulus_-like spine. The cephalon is about as wide as long, and eyes
have not yet been seen. The body consists of eleven segments and a
telson (Walcott says twelve and a telson but shows only eleven in the
figures). Nine of the segments, as in _Sidneyia_, are broad, the next
two narrow.
The ventral side of the cephalon has a long hypostoma, and five pairs
of appendages. The first pair are very long multi segmented antennules
and the next four pairs seem to be rather slender, spiniferous,
jointed endopodites. Whether or not gnathobases were present is not
shown by the figures, but owing to the long hypostoma the appendages
are grouped about the mouth. All the segments of the body, unless it
were the telson, seem to have borne appendages. On the anterior end,
they were clearly biramous (1912, p. 206, text fig. 10), and that they
were present along the body is shown by figure 2, plate 30, 1912.
The present state of knowledge of both these peculiar animals leaves
much to be desired. The indications are that the cephalic appendages
are not biramous, and that only one pair of antennæ, the first, are
developed as tactile organs. The thoracic appendages of _Emeraldella_
are biramous, and also possibly those of _Sidneyia_. In the latter,
the last two abdominal segments seem to have been without appendages,
while in _Emeraldella_ at least one branch of each appendage, and
possibly both, is retained.
These animals, which may be looked upon as the last survivors of an
order of pre-Cambrian arthropods, have the appearance of an
eurypterid, but their dominant characteristics are crustacean. The
features which suggest the Eurypterida are: elongate, obovate,
non-trilobate, tapering body; telson-like posterior segment; marginal,
compound, sessile eyes; claw-like third cephalic appendages; and, more
particularly, the general resemblance of the test to that of an
eurypterid like _Strabops_. In form, _Sidneyia_ agrees with the
theoretical prototype of the Eurypterida reconstructed by Clarke and
Ruedemann (Mem. 14, N. Y. State Mus., vol. 1, 1912, p. 124) in its
short wide head with marginal eyes, and its undifferentiated body.
There is, moreover, no differentiation of the postcephalic appendages.
The crustacean characteristics are seen in the presence of five,
instead of six, pairs of appendages on the head, the first of which
are multisegmented antennules, and in the biramous appendages on the
body of _Emeraldella_. It should be noted that these latter are
typically trilobitic, each consisting of an endopodite with six
segments and a setiferous exopodite.
Clarke and Ruedemann (1912, p. 406) have discussed _Sidneyia_ briefly,
and conclude:
It seems to us probable that the Limulava [_Sidneyia_ and
_Amiella_] as described are not eurypterids but constitute a
primitive order, though exhibiting some remarkable adaptive
features. This order possibly belongs to the Merostomata, but is
distinctly allied to the crustaceans in such important characters
as the structure of the legs and telson, and is therefore much
generalized.
The specialization of _Sidneyia_ consists in the remarkable
development of a highly complex claw on each of the third cephalic
appendages, and in the compound tail-fin, built up of the last segment
and one or more pairs of swimmerets. These two characteristics seem to
preclude the possibility of deriving the eurypterids from _Sidneyia_
itself, but it seems entirely within reason that they may have been
derived from another slightly less specialized member of the same
order.
That _Sidneyia_ is descended from any known trilobite seems highly
improbable, but that it was descended from the same ancestral stock as
the trilobites is, I believe, indicated by the presence of five pairs
of appendages on the cephalon and trilobitic legs on the abdomen.
=Molaria= and =Habelia.=
Other so-called Merostomata found by Walcott in the Middle Cambrian
are the genera _Molaria_ and _Habelia_, both referred to the Cambrian
family Aglaspidæ. These genera seem to conform with _Aglaspis_ of the
Upper Cambrian in having a trilobite-like cephalon without facial
sutures, a trilobite-like thorax of a small but variable (7-12) number
of segments, and a _Limulus_-like telson. Neither of them has yet been
fully described or figured, but (Walcott 1912 A, p. 202) _Habelia_
appears to have five pairs of cephalic appendages, the first two pairs
of which are multisegmented antennæ. The thoracic appendages are
likewise none too well known, but they appear to have been biramous.
The endopodites are better preserved than the exopodites, but in at
least one specimen of _Molaria_ the exopodites are conspicuous.
If these genera are properly described and figured, their appendages
are typically crustacean, and fundamentally in agreement with those of
_Marrella_. The relation to the Trilobita is evidently close, the
principal differences being the absence of facial sutures and the
presence of true antennæ. I am therefore transferring the Aglaspidæ
from the Merostomata to a new subclass under the Crustacea.
ARANEÆ.
The spiders have the head and thorax fused, the abdomen unsegmented
except in the most primitive suborder, and so appear even less
trilobite-like than the insects. The appendages likewise are highly
specialized. The cephalothorax bears six pairs of appendages, the
first of which are the pre-oral cheliceræ, while behind the mouth are
the pedipalpi and four pairs of ambulatory legs. The posterior pairs
of walking legs belong to the thorax, but the anterior ones are to be
homologized with the maxillæ of Crustacea, so that the spiders are
like the trilobites in having functional walking legs on the head.
The chief likenesses are, however, seen in the very young. On the germ
band there appear a pair of buds in front of the rudiments of the
cheliceræ which later unite to form the rostrum of the adult. At the
time these buds appear, the cheliceræ are post-oral, but afterward
move forward so that both rostrum and cheliceræ are in front of the
mouth. The rostrum is therefore the product of the union of the
antennules, and the cheliceræ are to be homologized with the antennæ.
There seems to be some doubt about the homology of the pedipalps with
the mandibles, as at least one investigator claims to have found
rudiments of a segment between the one bearing the cheliceræ and that
with the pedipalps.
Jaworowski (Zool. Anzeiger, 1891, p. 173, fig. 4) has figured the
pedipalp from the germ band of _Trochosa singoriensis_, and called
attention to the fact that it consists of a coxopodite and two
segmented branches which may be interpreted as exopodite and
endopodite. He designated as exopodite the longer branch which
persists in the adult, but since the ambulatory legs of Crustacea are
endopodites, that would seem a more likely interpretation. As the
figure is drawn, the so-called endopodite would appear to spring from
the proximal segment of the "exopodite." If the two terms were
interchanged, the homology with the limb of the trilobite or other
crustacean would be quite perfect.
In the young, the abdomen is segmented and the anterior segments
develop limb-buds, the first pair of which become the lung books and
the last two pairs the spinnerets of the adult. There seems to be some
question about the number of segments. Montgomery (Jour. Morphology,
vol. 20, 1909, p. 337). reviewing the literature, finds that from
eight to twelve have been seen in front of the anal segment. The
number seem to vary with the species studied. This of course suggests
connection with the anomomeristic trilobites.
The oldest true spiders are found in the Pennsylvanian, and several
genera are now known. The head and thorax are fused completely, but
the abdomen is distinctly segmented. Some of the Anthracomarti
resemble the trilobites more closely than do the Araneæ, as they lack
the constriction between the cephalothorax and abdomen. The spiders of
the Pennsylvanian have this constriction less perfectly developed than
do modern Araneæ, and occupy an intermediate position in this respect.
In the Anthracomarti, the pedipalpi are simple, pediform, and all the
appendages have very much the appearance of the coxopodites and
endopodites of trilobites. Cheliceræ are not known, and pleural lobes
are well developed in this group. Anthracomarti have not yet been
found in strata older than the Pennsylvanian, but they seem to be to a
certain extent intermediate between true spiders and the marine
arachnid.
Insecta.
Handlirsch (in several papers, most of which are collected in "Die
Fossilen Insekten," 1908) has attempted to show that all the
Arthropoda can be derived from the Trilobita, and has advocated the
view that the Insecta sprang directly from that group, without the
intervention of other tracheate stock. At first sight, this
transformation seems almost an impossibility, and the view does not
seem to have gained any great headway among entomologists in the
fourteen years since it was first promulgated. If an adult trilobite
be compared with an adult modern insect, few likenesses will be seen,
but when the trilobite is stripped of its specializations and compared
with the germ-band of a primitive insect, the theory begins to seem
more possible.
Handlirsch really presented very little specific evidence in favor of
his theory. In fact, one gets the impression that he has insisted on
only two points. Firstly, that the most ancient known insects, the
Palæodictyoptera, were amphibious, and their larvæ, which lived in
water, were very like the adult. Secondly, that the wings of the
Palæodictyoptera probably worked vertically only, and the two main
wings were homologous with rudimentary wing-like outgrowths on each
segment of the body. These outgrowths have the appearance of, and
might have been derived from, the pleural lobes of trilobites.
He figured (1908, p. 1305, fig. 7) a reconstructed larva of a
palæodictyopterid as having biramous limbs on each segment, but so far
as I can find, this figure is purely schematic, for there seems to be
no illustration or description of any such larva in the body of his
work.
That the insects arose directly from aquatic animals is of course
possible, and Handlirsch's first argument has considerable force. It
may, however, be purely a chance that the oldest insects now known to
us happen to be an amphibious tribe. The Palæodictyoptera are not yet
known to antedate the Pennsylvanian, but there can be no doubt that,
insects existed long before that time, and the fact that their remains
have not been found is good evidence that the pre-Pennsylvanian
insects were not aquatic. Comstock, who has recently investigated the
matter, does not believe that the Palæodictyoptera were amphibious
(The Wings of Insects, Ithaca, N. Y., 1918, p. 91).
The second argument, that wings arose from the pleural lobes of
trilobites, is exceedingly weak. Where most fully set forth (1907, p.
157), he suggests that trilobites may occasionally have left the
water, climbed a steep bank or a plant, and then glided back into
their native element, taking advantage of the broad flat shape to make
a comfortable and gentle descent! This sport apparently became so
engaging that the animal tried experiments with flexible wing tips,
eventually got the whole of the pleural lobes in a flexible condition,
and selected those of the second and third thoracic segments for
preservation, while discarding the remainder. The pleural lobes of
trilobites are not only too firmly joined to the axial portion of the
test to be easily transformed into movable organs, but they are
structurally too unlike the veined wings of insects to make the
suggestion of this derivation even worthy of consideration.
Tothill (1916) has recently reinvestigated the possible connection
between insects, chilopods, and trilobites, and, from the early
appearance of the spiracles in the young, came to the conclusion that
the insects were derived from terrestrial animals. He suggested that
they may have come through the chilopods from the trilobites. The
hypothetical ancestor of the insects, as restored by Tothill from the
evidence of embryology and comparative anatomy, is an animal more
easily derived from the Chilopoda than from the Trilobita. Five pairs
of appendages are present on the head, and the trunk is made up of
fourteen similar segments, each with a pair of walking limbs and a
pair of spiracles.
Only the maxillæ and maxillulæ are represented as biramous. If the
ancestor of the Insecta was, as seems possible, tracheate, this fact
alone would rule out the trilobites. Among tracheates, the Chilopoda
are certainly more closely allied to the Insecta than are any other
wingless forms. If the ancestors of the insects were not actually
chilopods, they may have been chilopod-like, and there can be little
doubt that both groups trace to the same stock.
As to the ancestry of the Chilopoda, it is probable that they had the
same origin as the other Arthropoda. Tothill has pointed out that in
the embryo of some chilopods there are rudiments of two pairs of
antennæ and that the two pairs of maxillæ and the maxillipeds are
biramous. This would point rather to the Haplopoda than directly to
the trilobites as possible ancestors, and may explain why the former
vanish so suddenly from the geological record after their brief
appearance in the Middle Cambrian. They may have gone on to the land.
There seem to be no insuperable obstacles to prevent the derivation,
indirectly, of the insects from some trilobite with numerous free
segments, and small pygidium. The antennules and pleural lobes must be
lost, the antennas and trunk limbs modified by loss of exopodites.
Wings and tracheæ must be acquired.
Handlirsch places the date of origin of the Insecta rather late, just
at the end of the Devonian and during the "Carboniferous." By that
time most families of trilobites had died out, so that the
possibilities of origin of new stocks were much diminished. If the
haplopod-chilopod-insect line is a better approximation to the truth,
then the divergence began in the Cambrian.
Chilopoda.
The adult chilopod lacks the antennules, and all of the other
appendages, with the exception of the maxillulæ, are uniramous. The
walking legs are similar to the endopodites of trilobites, and usually
have six or seven segments. The appendages are therefore such as could
be derived by modification of those of trilobites by the almost
complete loss of the exopodites and shortening of the endopodites of
the head. The position of the post-oral appendages, the posterior ones
outside those closest the mouth, is perhaps foreshadowed in the
arrangement of those of Triarthrus.
The Chilopoda differ from the Hexapoda in developing the antennæ
instead of the antennules as tactile organs, but this can not be used
with any great effect as an argument that the latter did not arise
from the ancestors of the former, since it is entirely possible that
in early Palæozoic times the pre-Chilopoda possessed two pairs of
antennæ. The first pair are still recognizable in the embryo of
certain species.
The oldest chilopods are species described by Scudder (Mem. Boston
Soc. Nat. Hist., vol. 4, 1890, p. 417, pl. 38) from the Pennsylvania!!
at Mazon Creek, Grundy County, Illinois. Only one of these, _Latzelia
primordialis_ Scudder (pl. 38 fig. 3), is at all well preserved. This
little animal, less than an inch long, had a depressed body, with a
median carina, exceedingly long slender legs, and about nineteen
segments. The head is very nearly obliterated.
Diplopoda.
The diplopods, especially the polydesmids with their lateral
outgrowths, often have a general appearance somewhat like that of a
trilobite, but on closer examination few likenesses are seen. The most
striking single feature of the group, the possession by each segment
of two pairs of appendages, is not in any way foreshadowed in the
trilobites, none of which shows any tendency toward a fusion of pairs
of adjacent segments. The antennules are short, antennæ absent,
mandibles and maxillulæ much modified, the latter possibly biramous,
and the maxillæ absent. The trunk appendages are very similar to those
of chilopods, and could readily be derived from the endopodites of
trilobites.
The oldest diplopods are found in the Silurian (Ludlow) and Devonian
(Lower Old Red) of Scotland, and three species belonging to two genera
are known. The oldest is _Archidesmus loganensis_ Peach (1889, p. 123,
pl. 4, fig. 4), and the Devonian species are _Archidesmus macnicoli_
Peach and _Kampecaris forfarensis_ Page (Peach 1882, p. 182, pl. 2,
fig. 2, 2a, and p. 179, pl. 2, figs. 1-1g). All of these species show
lateral expansions like the recent Polydesmidæ, and these of course
suggest the pleural lobes of trilobites. All three of the species are
simpler than any modern diplopod, for there is only a single pair of
appendages on each segment. No _foramina repugnatoria_ were observed,
and the eyes of _Kampecaris forfarensis_ as described are singularly
like those of a phacopid.
Peach says: "The eye itself is made up of numerous facets which are
arranged in oblique rows, the posterior end of each row being inclined
downwards and outwards, the facets being so numerous and so close
together that the eye simulates a compound one." There is also a
protecting ridge which somewhat resembles a palpebral lobe (1882, pl.
7, fig. la). Peach comments on the strength of the test, and from his
description it appears that it must have been preserved in the same
manner as the test of trilobites. It was punctate, and granules and
spines were also present. The presence of the lateral outgrowths in
these ancient specimens would seem to indicate that they are primitive
features, and may have been inherited. While possibly not homologous
with the pleural extensions of trilobites, they may be vestiges of
these structures.
The limbs are made up of seven segments which are circular in section
and expand at the distal end. The distal one bears one or two minute
spines. They are most readily compared with the endopodites of
_Isotelus_. The resemblance is, in fact, rather close. The sternal
plates are wider and the limbs of opposite sides further apart than in
modern diplopods. Except for one pair of antennæ, no cephalic
appendages are preserved.
While these specimens do not serve to connect the Diplopoda with the
Trilobita, they do show that most of the specializations of the former
originated since Lower Devonian times, and lead one to suspect that
the derivation from marine ancestors took place very early, perhaps in
the Cambrian. If no very close connection with the trilobites is
indicated, there is also nothing to show that the diplopods could not
have been derived from that group.
Primitive Characteristics of Trilobites.
TRILOBITES THE MOST PRIMITIVE ARTHROPODS.
The Arthropoda, to make the simplest possible definition, are
invertebrate animals with segmented body and appendages. The most
primitive arthropod would appear to be one composed of exactly similar
segments bearing exactly similar appendages, the segments of the
appendages themselves all similar to one another. It is highly
improbable that this most primitive arthropod imaginable will ever be
found, but after a survey of the whole phylum, it appears that the
simpler trilobites approximate it most closely.
That the trilobites are primitive is evidenced by the facts that they
have been placed at the bottom of the Crustacea by all authors and
claimed as the ancestors of that group by some; that Lankester derived
the Arachnida from them; and that Handlirsch has considered them the
progenitors of the whole arthropodan phylum.
Specializations among the Arthropoda, even among the free-living
forms, are so numerous that it would be difficult to make a complete
list of them. In discussing the principal groups, I have tried to show
that the essential structures can be explained as inherited from the
Trilobita, changed in form by explainable modifications, and that new
structures, not' present in the Trilobita, are of such a nature that
they might be acquired independently in even unrelated groups.
The chief objections to the derivation of the remainder of the
Crustacea from the trilobites have been: first, that the trilobites
had broad pleural extensions; second, that they had a large pygidium;
and lastly, that they had only one pair of tactile antennæ.
It has now been pointed out that many modern Crustacea have pleural
extensions, but that they usually bend down at the sides of the body,
and also that in the trilobites and more especially in _Marrella_,
there was a tendency toward the degeneration of the pleural lobes. A
glance at the Mesonacidæ or Paradoxidæ should be convincing proof that
in some trilobites the pygidium is reduced to a very small plate.
In regard to the second antennæ standard text-books contain statements
which are actually surprising. A compilation shows that the antennæ
are entirely uniramous in but a very few suborders, chiefly among the
Malacostraca; that they are biramous with both exopodite and
endopodite well developed in most Copepoda, Ostracoda, and
Branchiopoda; and that the exopodite, although reduced in size, still
has a function in some suborders of the Malacostraca. The Crustacea
could not possibly be derived from an ancestor with two pairs of
uniramous antennæ.
Although I have defended the trilobites, perhaps with some warmth,
from the imputation that they were Arachnida, my argument does not
apply in the opposite direction, and I believe Lankester was right in
deriving the Arachnida from them. If the number of appendages in front
of the mouth is fundamental, then the trilobites were generalized,
primitive, and capable of giving rise to both' Crustacea and
Arachnida. As shown on a previous page (p. 119), the "connecting
links" so far found tend to disprove rather than to prove the thesis,
but the present finds should be looked upon as only the harbingers of
the greater ones which are sure to come.
LIMBS OF TRILOBITES PRIMITIVE.
The general presence, in an adult or larva, of some sort of biramous
limbs throughout the whole class Crustacea has led most zoologists to
expect such a limb in the most primitive crustaceans, and apparently
the appendage of the trilobite satisfies the expectation. It is well,
perhaps, as a test, to consider whether by modification this limb
could produce the various types of limbs seen in other members of the
class. In the first place, it is necessary to have clearly in mind the
peculiarities of the appendage to be discussed.
It should first of all be remembered that the limb is articulated with
the dorsal skeleton in a manner which is very peculiar for a
crustacean. The coxopodite swings on a sort of ball-and-socket joint,
and at the outer end both the exopodite and the basipodite articulate
with it. Since the exopodite articulates with the basipodite as well
as with the coxopodite, the two branches are closely connected with
one another and there is little individual freedom of movement. This
is, of course, a necessary consequence of their articulation with a
segment which is itself too freely movable to provide a solid base for
attachment of muscles. The relation of the appendifer, coxopodite, and
two rami is here shown diagrammatically (fig. 33), the exopodite
branching off from the proximal end of the basipodite at the junction
with the coxopodite.
In all trilobites the endopodite consists of six segments, and the
coxopodite of a single segment the inner end of which is prolonged as
an endobase. There does not seem to be any variation from this plan in
the subclass, although individual segments are variously modified. The
exopodites are more variable, but all consist of a flattened shaft
with setæ on one margin. No other organs such as accessory gills,
swimming plates, or brood pouches have yet been found attached to the
appendages, the evidence for the existence of the various epipodites
and exites described by Walcott being unsatisfactory (see p. 23).
[Illustration: Fig. 33.--Diagrammatic representation of an appendage
of the anterior end of the thorax of _Triarthrus becki_ Green, to show
relation of exopodite and endopodite to each other and to the
coxopodite. Much enlarged.]
In the Ostracoda the appendages are highly variable, but it is easily
seen that they are modifications of a limb which is fundamentally
biramous. In most species, both exopodite and endopodite suffer
reduction. The exopodite springs from the basipodite and that segment
is closely joined to the coxopodite, producing a protopodite. In some
cases the original segments of the endopodites fuse to form a stiff
rod. While highly diversified, these appendages are very
trilobite-like, and some Ostracoda even have biramous antennæ.
The non-parasitic Copepoda have limbs exceedingly like those of
trilobites. Many of them are biramous, the endopodites sometimes
retaining the primitive six segments. Coxopodite and basipodite are
generally united, and endopodite and exopodite variously modified.
Like some of the Ostracoda, the more primitive Copepoda have biramous
antennæ.
As would be expected, the appendages of the Cirripedia are much
modified, although those of the nauplius are typical. The thoracic
appendages of many are biramous, but both branches are multisegmented.
In the modern Malacostraca the ground plan of the appendages is
biramous, but in most orders they are much modified. In many, however,
the appendages of some part of the body are biramous, and in many the
endopodites show the typical six segments. From the coxopodites arise
epipodites, some of which assist in swimming, and some in respiration.
Because of the many instances in which such extra growths arise, and
because of the form of the appendages of the Branchiopoda, it has
been suggested that the primitive crustacean leg must have been more
complex than that of the trilobite. In looking over the Malacostraca,
however, one is struck by the fact that epipodites generally arise
where the exopodites have become aborted or are poorly developed, and
seem largely to replace them. The coxopodite and basipodite are
usually fused to form a protopodite, and a third segment is sometimes
present in the proximal part of the appendage.
In the Branchiopoda are found the most complex crustacean limbs, and
the ones most difficult to homologize with those of trilobites. In
recent years, Lankester's homologies of the parts of the limbs of
_Apus_ with those of the Malacostraca have been quite generally
accepted, and the appendages of the former considered primitive.
Now that it is known that the Branchiopoda of the Middle Cambrian
(_Burgessia_ _et at._) had simple trilobite-like appendages, it
becomes necessary to exactly reverse the opinion in this matter. The
same homologies stand, but the thoracic limbs of _Apus_ must be looked
upon as highly specialized instead of primitive.
[Illustration: Fig. 34.--One of the appendages of the anterior part of
the trunk of _Apus_, showing the endites (beneath) and exites (above).
The proximal endite forms a gnathobase which is not homologous with
the gnathobase (or endobase) of the trilobite. Copied from Lankester.
Much enlarged.]
Lankester (Jour. Micros. Sci., vol. 21, 1881) pointed out that the
axial part of the thoracic limb of _Apus_ (fig. 34) is homologous with
the protopodite in the higher Crustacea, that the two terminal endites
corresponded to the exopodite and endopodite, and that the other
endites and exites were outgrowths from the protopodite analogous
to the epipodites of Malacostraca. There seems to be no objection
to retaining this interpretation, but with the meaning that both
endopodite and exopodite are much reduced, and their functions
transferred to numerous outgrowths of the protopodite. One of the
endites grows inward to form an endobase, the whole limb showing an
attempt to return to the ancestral condition of the trilobite. The
limbs of some other branchiopods are not so easy to understand, but
students of the Crustacea seem to have worked out a fairly
satisfactory comparison between them and _Apus_.
The discovery that the ancestral Branchiopoda had simple biramous
appendages instead of the rather complex phyllopodan type is another
case in which the theory of "recapitulation" has proved to hold. It
had already been observed that in ontogeny the biramous limb preceded
the phyllopodan, but so strong has been the belief in the primitive
character of the Apodidæ that the obvious suggestion has been ignored.
Even in such highly specialized Malacostraca as the hermit crabs the
development of certain of the limbs illustrates the change from the
schizopodal to the phyllopodan type, and Thompson (Proc. Boston Soc.
Nat. Hist., vol. 31, 1903, pl. 5, fig. 12) has published an especially
good series of drawings showing the first maxilliped. In the first to
fourth zoeæ the limb is biramous but in the glaucothoe a pair of broad
processes grow out from the protopodite, while the exopodite and
particularly the endopodite become greatly reduced. In the adult the
endopodite is a mere vestige, while the flat outgrowths from the
protopodite have become very large and bear setæ.
_Summary._